A. Field of the Invention
This invention concerns an electrically operated fuel ignitor for use with appliances utilizing gaseous fuel. The invention more particularly concerns electrically operated gas ignitors for use with appliances such as gas operated stoves.
B. History of the Prior Art
In the prior art, appliances utilizing gaseous fuel such as hydrocarbon gases were ignited by means of a pilot light which was usually a small flame maintained by burning hydrocarbon gas. The gas fired pilot light usually consumed from about 10 to about 40 percent of the gas used by the appliance and the gas consumed by the pilot light provided no other benefit except providing for ignition of major fuel burners. Otherwise, the gas operating the pilot light was wasted. An advance was made in the prior art when electrical ignitors were developed. These ignitors either became or were maintained above ignition temperature by an electrical current to ignite the fuel burners. Such prior art electrical ignitors however were generally of short life, consumed excessive amounts of electrical current or reached ignition temperature too slowly for range top application. In the prior art there were two general types of electrical gas ignitors one of which ignited the gas by means of an electrical spark between two electrodes. This type of ignitor requires high voltage electrical energy and is subject to failure due to pitting and burning of the electrodes by the spark. Such spark type ignitors further require high voltage transformers, capacitors and other complicated electronic components to make the spark ignitor functional, particularly when repeated sparking is required in the event that ignition of the fuel does not promptly occur or in the event that for some reason combustion of the fuel stops.
The second type of ignitor for gaseous fuels are the hot surface gas ignitors which comprise a body which is heated above ignition temperature of the gas when sufficient electrical current passes through the body of the ignitor. In the prior art hot surface gas ignitors were made from numerous materials including platinum, molybdenum disilicide and silicon carbide. In order for a hot surface gas ignitor to be satisfactory the material of its construction must possess adequate oxidation resistance, must have suitable electrical properties and must be thermal shock resistant. In addition the ignitor must reach ignition temperature rapidly. It is also desirable that the material of the construction of the gas ignitor not be an insulator yet have a large high temperature volume resistivity so that a large voltage drop occurs and only a low current flows. Such low current flow minimizes the cost of the power supply for the ignitor and reduces the cost of electrical energy. Platinum and molybdenum disilicide have metallic conduction and therefore have unsuitably low electrical resistance. Silicon carbide however, has an electrical resistivity which is substantially higher than platinum and molybdenum disilicide. Prior art silicon carbide hot surface gas ignitors were not, however, suitable since high voltages were required to overcome the cold electrical resistance of the silicon carbide, relative to the voltage needed to maintain ignition temperature at the ignitor's hot resistance. Such prior art silicon carbide ignitors used 40 or more watts of energy to heat the ignitor above the ignition temperature of the gas. In addition, such ignitors were very slow, eg. over 6 seconds and usually over 10 seconds, before gas ignition temperature was reached. It has been found that this slow ignition time was partially due to the large size of the ignitor, ie. over 40 watts, to raise the temperature of the ignitor body over the ignition temperature of the gas and furthermore such prior art carbide ignitors were slow since the ignitors had a high cold resistance, relative to its hot resistance, which had to be overcome before sufficient electrical current could flow through the ignitor to heat it above the ignition temperature of the fuel. The cold resistance of such prior art ignitors was sometimes so high relative to its hot resistance that when sufficient voltage was applied to overcome the cold resistance, that same voltage would force unacceptably large amounts of current through the heated ignitor as a result of its lower hot resistance. Those large amounts of current would then result in overheating of the ignitor thus damaging it, destroying it or reducing its useful life. Attempts at reducing the size of such silicon carbide hot surface ignitors in order to reduce voltage and energy requirements were unsuccessful. While reducing the length of the ignitor reduces cold resistance thus reducing initial voltage requirements, the result is an ignitor which is either much too slow for most practical applications or would overheat. This is true because reducing the length of the ignitor did not change the ratio of cold to hot resistance of the ignitor. Furthermore, prior art silicon carbide compositions had insufficient strength at the small sizes required unless additives were present. Such additives incorporated into the silicon carbide usually resulted in an undesirable altering of the electrical properties of the ignitor by increasing the ratio of cold to hot resistance which in turn increased heat up time, decreased the life of the ignitor unless complex circuitry is used to regulate voltage, and increased the amount of electrical potential required to heat the ignitor to a temperature above the ignition temperature of the gas.
Prior art methods of manufacturing silicon carbide ignitors included reaction sintering which consists of siliconizing a carbon-silicon carbide mixture. The result of such reaction sintering was the presence of free silicon which undesirably alters the properties of the ignitor and was difficult to control. The free silicon could be removed in a high temperature furnace which resulted in a coarse microstructure in the ignitor which in turn resulted in an ignitor having insufficient strength. In the prior art attempts have been made to manufacture such ignitors by sintering silicon carbide through the use of boron containing additives which were necessary to achieve sufficient strength. The boron is again detrimental to the electrical properties of the ignitor since an unacceptably high ratio of cold to hot resistivity results.
For the above reasons, prior art silicon carbide compositions were not wholly suitable for the manufacture of hot surface ignitors.